1. Field of the Invention
The present invention relates to a thread forming method, a thread forming device and a thread forming tool for forming a thread portion for fastening other plate member or the like to a metal plate.
2. Related Art
In a related art, as a method for screw cramping-a metal plate used in a vehicle or the like, there is used a thread portion forming method for forming a thread portion at a predetermined position of the metal plate by subjecting the metal plate to burring and threading.
JP-A-05-253747 discloses a tapping tool including a boring portion for forming a pilot hole by burring by breaking a thin plate while plastically deforming the thin plate by friction heat by pressing and rotating by a front end thereof in a taper shape, and a tapping portion for forming an internal thread at the pilot hole formed by the boring portion and forming a dull processing face at the boring portion.
However, according to the constitution of JP-A-05-253747, as shown by FIG. 16A, in burring, a large portion of a press force exerted from a boring portion 2 to a thin plate 3 is applied in a direction indicated by an arrow mark in FIG. 16A, that is, a skewed lower direction. Therefore, as shown by FIG. 14, there poses a problem that a plate thickness at a front end portion of a thread portion 1 is reduced, and a strength of an internal thread is reduced. Further, when a press speed of a tool is constituted by a high speed (for example, about 10 mm/sec), as shown by FIG. 15, a shape of the thread portion 1 becomes nonuniform, a front end thereof is broken. Therefore, the press speed of the tool needs to be constituted by a low speed (for example, about 1.5 mm/sec) to pose a problem that a forming time period is increased.